In a technology for removing ionic substances in sea water, brackish water, and the like, in recent years, a method of separation by a separation membrane element is widely used as a process for energy conservation and natural resource saving. Separation membranes used for the method of separation by a separation membrane element are divided into a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, a reverse osmosis membrane, and a forward osmosis membrane in terms of its pore diameter or a separation function. These membranes are used, for example, in the production of drinking water from sea water, brackish water, water containing toxic substances, and the like, as well as in the production of industrial ultra-pure water, wastewater treatment, and recovery of valuables. The membranes used are selected depending on the target component to be separated as well as the separation performance.
Various forms exist for the separation membrane element, but they are common in that raw fluid is supplied to one surface of a separation membrane and permeate fluid is obtained from the other surface. The separation membrane element is configured by including many separation membranes bundled so that a membrane area per separation membrane element is increased, that is, so that an amount of permeate fluid to be obtained per separation membrane element is increased. As the separation membrane element, various forms such as a spiral type, a hollow fiber type, a plate and frame type, a rotating flat-sheet membrane type and an integrated flat-sheet membrane type are proposed according to uses and purposes.
For example, a spiral separation membrane element is widely used for reverse osmosis-filtration. The spiral separation membrane element includes with a central tube and a laminate wound around the central tube. The laminate is formed by laminating a channel material on the feed side for supplying raw fluid to the surface of the separation membrane, a separation membrane for separating components in the raw fluid, and a channel material on the permeate side for guiding the fluid on the permeate side, which permeates the separation membrane to be separated from the fluid on the feed side, to the central tube. The spiral separation membrane element is preferably used since it can provide pressure for the raw fluid and therefore a large amount of the permeate fluid can be drawn out.
In the spiral separation membrane element, in general, a polymer net is mainly used as a channel material on the feed side for the formation of a channel on the feed side fluid. Further, as the separation membrane, a laminate type separation membrane is used. The laminate type separation membrane is a separation membrane comprising a separation function layer of a crosslinkable high molecular weight compound such as polyamide, a porous resin layer of a high molecular weight compound such as polysulfone, and a nonwoven fabric of a high molecular weight compound such as polyethylene terephthalate, which are disposed in this order from the feed side to the permeate side. Further, a knit fabric member referred to as “tricot,” which has a smaller channel interval than the channel material on the feed side, is used for the channel material on the permeate side for the purpose of preventing falling of the separation membrane and forming the flow path on the permeate side.
In recent years, improvement in performance of the membrane element has been demanded because of increasing requirement for reducing the cost of water production. To improve separation performance of the separation membrane element and to increase the amount of the permeate fluid produced per unit time, improvement in performance of members of the separation membrane element such as respective channel members has been proposed.
Specifically, JP 2006-247453 A proposes an element comprising a sheet member provided with projections and depressions as the channel material on the permeate side. JP 2010-099590 A proposes an element which does not require the channel material such as a net on the feed side or the channel material such as tricot on the permeate side by disposing a sheet-like separation membrane comprising a porous support having projections and depressions formed thereon and a layer having separation activity.
However, the separation membrane elements as described above are not sufficient in improving their performance, particularly in improving the stability of the separation performance over long term operations.
Thus, it could be helpful to provide a separation membrane and a separation membrane element which can stabilize separation/removal performance at the time when the separation membrane element is operated by particularly applying high pressure.